Vim auto-indenting xml strangely

Question

In vim (neovim), XML code is indenting strangely when attributes are put on their own lines, like so:

<test
    first="text"
    second="more text"
    third="last">
<foo
    a="a"
    b="b">
<bar />
</foo>
</test>

When I expect, and desire, this:

<test
    first="text"
    second="more text"
    third="last">
    <foo
        a="a"
        b="b">
        <bar />
    </foo>
</test>

This feels like none other than a bug to me. I could not find a plugin to address this. Perhaps there is something that I could add to my init.vim to correct this behavior?

EDIT: The patch below almost fixes this issue, but does not correctly handle the following case:

<test
    first="text"
    second="more text"
    third="last">
    <foobar>
    </foobar>
    <foobar>
    </foobar>
</test>

EDIT 2: Removing the + shiftwidth() on line 90 solves all but this case:

<test
    first="text">
    <foobar>
    </foobar>
</test>

<foobar> should indent as shown above, because the tag spanning the first two lines is an opening tag, indicating that the next lines should be indented.

EDIT 3: For some reason, <foo is indenting twice with the patch. Expected:

<test>
    <foo
        a="a">
    </foo>
</test>

I know regex enough to play with it, but vim script makes this tricky.

EDIT 4: Expected output should be:

<test>
    <foo
        a="a" />
    <foobar
        a="a" >
        <test>
        </test>
    </foobar>
</test>

/> should not indent the next line.

Answer 1

I'm not knowledgeable enough about XML to determine whether it's expected behavior nor can do proper tests, so I don't know if my solution is the best.

But if you suspect it to be a bug, then the best action would be to report¹ it on the official GitHub repository of XML runtime files: chrisbra/vim-xml-runtime.

---

I managed to get your expected results by applying this patch²:

@@ -81,11 +81,16 @@
 endfun

 " [-- return the sum of indents of a:lnum --]
-fun! <SID>XmlIndentSum(line, style, add)
+fun! <SID>XmlIndentSum(line, style, add, ...)
     if <SID>IsXMLContinuation(a:line) && a:style == 0 && !<SID>IsXMLEmptyClosingTag(a:line)
         " no complete tag, add one additional indent level
         " but only for the current line
         return a:add + shiftwidth()
+    elseif a:0 && a:line =~ '^\s*<[^/]' && a:1 =~ '^\s*<[^/]' && a:1 !~ '>\s*$'
+                \ && getline(prevnonblank(a:2-1)) !~ '/\s*>\s*$'
+        return a:add + shiftwidth()
+    elseif a:0 && a:line =~ '^\s*</' && a:1 =~ '^\s*<' && a:1 !~ '>\s*$'
+        return a:add
     elseif <SID>HasNoTagEnd(a:line)
         " no complete tag, return initial indent
         return a:add
@@ -156,7 +161,7 @@
     " Get indent from previous tag line
     let ind = <SID>XmlIndentSum(pline, -1, pind)
     " Determine indent from current line
-    let ind = <SID>XmlIndentSum(curline, 0, ind)
+    let ind = <SID>XmlIndentSum(curline, 0, ind, pline, a:lnum)
     return ind
 endfun

Just copy the indent file to ~/.vim/indent/xml.vim ³ and apply the change.

---

¹ You may link this answer in the issue
² Which is of quality of just a temporary hack
³ I believe it's $XDG_CONFIG_HOME/nvim/indent/xml.vim in case of Neovim